Error when deleting database that doesn't exist on disk anymore

[max-schaefer]

Describe the bug
When trying to delete a database from the UI that doesn't exist on disk anymore, I sometimes get "Cannot read property 'belongsToSourceArchiveExplorerUri' of undefined", possibly originating here (though this is a guess, since I don't have a stack trace).

To Reproduce

• Create a new database, for example using codeql test run
• Import it
• Exit VSCode
• Remove the database
• Start VSCode again; the database now displays a red cross next to it.
• Try removing the database; the log will show a message about "Removing workspace folder at index -1", possibly caused by the typo fixed in Fix dubious index check #692. The database is not removed.
• Try removing the database again; the log will show the message mentioned above.

Expected behavior
It should delete the database.

[aeisenberg]

This issue is fixed by #692.